How Does Lithographic Printing Work?

Wondering how lithographic printing works? Here is some information from working litho printers about the process.
 

The Process

In lithography the plates are covered in a light sensitive (photosensitive) emulsion, formed from suspension of two chemicals.

There are two alternative ways that can be used to create plates. Negative or positive plates are exposed to a UV light source with negative or positive film on top of the plate.

The modern method is to take the customers PDF file and use a computer to transfer the image direct to plate from the computer ,missing out the process of negative or positive film. This process is called “CTP” Computer To Plate.

This then creates a photographic negative or positive image on the plate. The plate then has to have the excess unexposed emulsion chemically removed. The final step is to wash the chemicals off the plate to leave the image required to print.

The image that is left on the printing plate is rendered grease receptive and the non printing area on the plate is rendered hydrophilic. In simple terms the image left on the plate will ink up and the dampening system on the press will put the water on to the plate where there is no image. The image on the plate then rotates and transfers its image to the rubber compound known as the blanket. The blanket is then lowered on to the paper web. A clear sharp dry ink image is then printed on the paper.

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